Patent GB-1203366-A (GB)
Title: Electro-hydraulic servo control
IPC: F15B21/02
Filed: 1968-04-04
Abstract: 1,203,366. Injection moulding. FELLOWS GEAR SHAPER CO. 4 April, 1968 [30 Oct., 1967], No. 16354/68. Heading B5A. An injection moulding machine which is hydraulically operated to inject a plastics material into a mould cavity and to press and clamp the material within the cavity is associated with an hydraulic system for automatically controlling the direction and rate of flow of hydraulic fluid to the machine and thereby its sequence of operation. The system comprises a closed loop of conduits with a reversible pump 32 for carrying the fluid to all parts of the system, and a servo control unit 50 for actuating the pump and dictating the rate and direction of flow from it in accordance with a programmed sequence. The moulding machine may include mould halves 10, 12, Fig. 2, urged together by an hydraulic motor 22 connected in the closed hydraulic loop and a pivoted linkage 28; the injection cylinder 16 is moved axially to connect with the closed mould by hydraulic motors 18, 20, and the injector impeller is driven by hydraulic motors 66, 68, Fig. 3. Reversal of fluid flow by the pump 32 reverses the movement of motors 18, 20 and 22. The pump may be of the kind comprising a set of pistons 104, Fig. 4, within a rotary member 100 driven by a shaft 108; the piston ends are engaged by a rotary plate 110 within a non-rotary yoke 106 pivotally adjustable such that in the position indicated fluid is pumped to the line 36, and when the yoke is adjusted to lie on the axis C-C fluid will be pumped in the opposite direction in the loop through line 34. Intermediate yoke positions produce variable pumping pressures. Yoke adjustment is effected by a servo control unit 50 which includes a piston 118, Fig. 5, connected to a rod 116 attached to the yoke, and hydraulic fluid is fed to either side of the piston under the control of a solenoid operated valve 120 with a pivoted closure member 130. High pressure fluid is fed to the unit at 122 and passes to one side of the piston via a passage 124, and to the other side when the valve 120 is closed; the piston is then in the position indicated in Fig. 5 causing the pump to deliver through line 34 with consequent closure of the mould halves. By opening the valve i.e. effecting upward pivoting of the member 130 by a spring 132, pressure at the right of the piston is released causing movement of the piston to the right thereby pivoting the pump yoke to the position of Fig. 4 and causing fluid flow in the loop to be reversed. With the piston in the position of Fig. 5 and the valve 120 closed a safety valve may be operated to release the pressure on the right hand side of the piston to effect pump reversal. The motor M, Fig. 3, which drives the reversible pump also drives a variable displacement pump 40 which maintains a sufficient pressure in the line 34, and a fixed displacement pump 44 to supply all parts of the system with fluid from a supply 42. The system includes a number of solenoid operated valves &c. controlling fluid flow throughout the system in accordance with a programmed sequence relayed from an electrical control system. This includes means for comparing actual pump perforance with the performance commanded by the unit 50, and for correcting any discrepancies by adjusting signals sent to the unit and hence the pump operation. For this purpose the pump carries a feed back potentiometer 142, the adjustment arm 146 of which is moved by its being connected to a bracket 144 on the connecting rod 116. The electric circuit shown in Fig. 7 includes a section 200 with a number of adjustable potentiometers 222-228 to command a plurality of voltages corresponding to the number of sequential positions obtainable by the pump yoke; a logic circuitry selectively connects the voltages to an output 232 supplying an amplifier 238 connected to the valve 120. Feed back from the potentiometer 142 connects with a summing node 234. The circuit also includes a pressure sensing and override section 202 including adjustable potentiometers 250-254 giving voltages corresponding to a number of predetermined maximum pressures required for the hydraulic system. They are fed to a logic circuitry 256 which selectively feeds an output 258 supplying an amplifier 260. The wiper of a feed back potentiometer 262, which is operated by pressure in the loop is connected to the amplifier 260 whose output is connected to the summing node 234 via lead 272. Pressure increase above a predetermined maximum produces an output of amplifier 260 which affects the output of amplifier 238 to thereby operate the valve 120 and move the piston 118 and hence the pump yoke to reduce the pump output.